Add masking

[stuarta0]

PMVS2 supports masks and so does Blender. We should add masking to the PMVS output.

From the PMVS documentation:

Segmentation masks can be optionally given to the program. A segmentation mask must be given in pgm file and stored under root/masks/ directory. Their names must be again 00000000.pgm, 00000001.pgm, ... A pgm file must be in P5 (binary grey-scale) format and there should be no comments. If the intensity of a pixel is less than 127, the pixel is treated as background, and if the intensity is more than 127, the pixel is treaded as foreground. Intuitively, background should appear black and foreground should appear white. If files do not exist in the directory, the program just ignores them. You do not have to place pgm files for all the input images. You can, for example, put only 0000001.pgm and 00000002.pgm to specify mask images for these 2 images.

[stuarta0]

https://en.wikipedia.org/wiki/Netpbm_format